-How long have they been vending
-Are they actually selling quality or shit
-From which country are they shipping
Some countries are just way easier to make good money than others. Especially countries like Australia where it's generally hard to source quality.
I've noticed that Australian vendors with decent gear are almost always successful.
Countries like the Netherlands or Germany it's relatively easy to get drugs so there is a lot of competition. So you will need to be better than the other vendors to succeed.
Also keep in mind that building a good business takes time. You need both new customers and returning ones. So I'd say this will take maybe 6 months or so to build.
Lol I remember hearing that the top 10 vendors on a given market make %70 percent of the sales volume, but I heard it in 2016/17 so it's likely different today.